This application relates to and incorporates herein by reference Japanese Patent Application No. 2000-122857 filed on Apr. 24, 2000.
1. Field of the Invention
The present invention relates to a filter in which a plurality of resonators are formed circularly around the center of a substrate.
2. Related Art
In some conventional filters, resonators are formed in a hairpin pattern or forward-coupled pattern. The hairpin type is disclosed in, for instance, IEICE Transactions on Electronics, Vol. E82-C No. Jul. 7, 1999, xe2x80x9cHigh-temperature Superconducting Receiving Filtering Subsystem for Mobile Telecommunication Base Stationxe2x80x9d of Appl. Phys. Lett. 71(26), Dec. 29, 1997, xe2x80x9cMicrowave intermodulation in thin film high-Tc superconducting microstrip hairpin resonators : Experiment and theory.xe2x80x9d The forward-coupled type is disclosed in, for instance, Appl. Phys. Lett. 69(4), Jul. 22, 1996, xe2x80x9cHigh power failure of superconducting microwave filters: Investigation by means of thermal imaging.xe2x80x9d
The filters formed with the above resonator patterns require large substrates which extend in a lateral direction, when formed in multiple stages, or have only a limited number of stages in a fixed area. Further, it is difficult to regulate the coefficient or ratio of cross coupling which occurs between resonators other than the adjacent resonators in the above resonator patterns.
It is therefore an object of the present invention to provide a filter in which a plurality of resonators can be formed in a limited area of a substrate.
It is another object of the present invention to provide a filter in which the coefficient of coupling between resonators can be regulated with ease.
It is a further object of the present invention to provide a filter in which notches are provided at both edges of a passband for a sharp filtering or cut-off response.
According to the present invention, a filter for filtering electrical signals comprises a substrate and a plurality of resonators formed circularly on the substrate to surround the center of the substrate.
Each resonator has an arcuate part at a radially outermost part and a pair of linear parts extending from ends of the arcuate part in a radially inward direction and the arcuate part is located at the same distance from the center of the substrate. Thus, the number of the resonators provided in a limited area may be increased. Further, the linear parts of each resonator have different lengths from each other thereby to provide a sharp decrease of gain at edges of the passband in a filtering response.